1. Field of the Invention
The present invention relates to a method for the production of silicon from silyl halides.
2. The Prior Art
In the state of the art, different methods for the production of high-purity silicon are known. In an industrially established process, trichlorosilane HSiCl3 is used, which is thermally decomposed on a hot substrate, in the presence of hydrogen. The decomposition temperatures lie in the range of 800-1300° C., for example in the method described in the patent DE 1061593, which is known as the “Siemens process.” A decisive disadvantage of this method of procedure is the low conversion rate of the trichlorosilane used, which makes it necessary to work with large excess amounts of hydrogen, at 3.5 to 16 times the amount in comparison to stoichiometric use, in order to achieve a conversion of 20-40% of the trichlorosilane to silicon. Another disadvantage is that trichlorosilane must be produced, in cost-intensive manner, from metallurgical low-purity silicon and hydrogen chloride HCl (K. Hata, S. Nakamura, S. Yasuda, Japan. Tohoku Daigaku Senko Seiren Kenkyusho Iho 23(1) (1967) 45-54; GB 883326) or from tetrachlorosilane SiCl4 and hydrogen (U.S. Pat. No. 4,836,997; U.S. Pat. No. 3,933,985; U.S. Pat. No. 4,309,259). The latter method only achieves a low yield. In addition, the trichlorosilane introduced into the process is converted into SiCl4, to a great extent, during the precipitation of silicon, thereby making it impossible to simply return the chlorosilane into the process.
In a variant of this process as disclosed in DE 1061593, for example, the cheaper tetrachlorosilane is used as the silicon source, but even lower yields of silicon are obtained, with even greater hydrogen excess.
The precipitation methods of solid silicon from silyl halides at high temperatures have the common disadvantage that particularly these high temperatures promote a reverse reaction of the precipitated silicon with HCl that is also formed during the precipitation, forming silyl halides. A method disclosed in U.S. Pat. No. 3,625,846 counters this circumstance by means of intensive cooling of the product gases.
A starting compound in the case of which no equilibrium with by-products can occur during the precipitation of silicon is monosilane SiH4. However, this product must be produced in cost-intensive manner, for example from trichiorosilane (DE 2,507,864).
According to a more recent method, which is described in DE 1982587 C1, chlorosilanes are hydrogenated with hydrogen, to form monosilane, by means of stoichiometric amounts of alkali metal in salt melts.
The required high activation energies for the reaction of chlorosilanes with hydrogen are furthermore made available, in the state of the art, by means of the use of plasmas. Thus, for example, a capacitatively coupled plasma is utilized in GB 892014, for the decomposition of SiCl4/H2 mixtures, in order to precipitate silicon on hot surfaces (several hundred ° C.). Chlorosilane/hydrogen mixtures are also converted to melted silicon, in U.S. Pat. No. 4,102,985 and U.S. Pat. No. 4,102,764, using an electric arc discharge, at normal pressure.
Inductively coupled plasmas are also described in the state of the art; for example, the excitation of an SiF4/H2/Ar gas mixture to produce a gas discharge by means of an induction coil is described in US 2004/0250764 A1. The resulting silicon precipitates on silicon particles that are passed through the plasma zone.
A method described in GB 851290 precipitates elemental silicon by means of the action of atomic hydrogen on SiCl4, in the manner of a remote plasma source. For this purpose, the atomic hydrogen is generated by means of an electric discharge (50 Hz-100 MHz), at a pressure of 1 Torr, and the silyl halide is subsequently metered in through a nozzle.
In GB 823383, Si droplets are precipitated onto the electrodes by means of the action of an electric arc between electrodes. The electrodes are slowly drawn apart from one another, to the extent that the silicon is growing. Furthermore, microwave radiation for plasma generation in the production of Si is described, whereby very energy-rich microwave pulses of 1 MW power are used in U.S. Pat. No. 2,945,797, in order to achieve coupling-in of the radiation. There, contamination of the reactor wall with silicon is also mentioned, and this is supposed to be countered by means of intensive cooling.
The precipitation of crystalline silicon under reduced pressure, by means of microwave discharge at low power, in H2 that is mixed with 5% SiCl4, is described in the literature (P. M. Jeffers, S. H. Bauer, J. Non-Cryst. Solids 57 (1983) 189-193). U.S. Pat. No. 4,908,330 discloses the production of thin films of silicon at less than 1 Torr pressure, by means of the reduction of SiF4/Si2F6 with atomic hydrogen, which is generated by means of a microwave discharge in a separate plasma chamber (remote plasma).
Microwave radiation is also used in U.S. Pat. No. 4,786,477, U.S. Pat. No. 4,416,913, and U.S. Pat. No. 5,374,413, in order to achieve Si precipitation. In these methods, however, the purely thermal effect of the radiation on glowing silicon is utilized in order to heat Si particles to a high temperature, without any plasma formation taking place.
Another form of silicon that can be precipitated by means of plasma discharges is the so-called amorphous silicon, which usually still contains certain amounts of other elements (H, Cl, F, etc.). A method is known from the literature, in which an amorphous silicon is precipitated by means of an electric glow discharge in a gas mixture SiCl4/H2/He, under normal pressure, which mixture contains not only H but also about 1% Cl (O. H. Giraldo, W. S. Willis, M. Marquez, S. L. Suib, Y. Hayashi, H. Matsumoto, Chem. Mater. 10 (1998) 366-371).
Finally, the transition to polysilanes/polysilylenes is made with an increasing content of halogen and hydrogen in the silicon that is produced; in these, only two of the four possible valences are saturated off by means of bonds to additional Si atoms, on the average. Chlorinated polysilanes are produced in targeted manner, for example as described in JP 62143814, by means of the conversion of elemental Si with chlorine, in inert organic solvents, or as disclosed in JP 59195519, by means of reaction of silicides with chlorine. U.S. Pat. No. 4,374,182, EP 0282037 A2, JP 1197309, and JP 1192716 disclose the formation of silicon from chlorinated polysilanes SinCl2n+2 by means of disproportionation or reduction with H2, at elevated temperature.
High-purity silicon is also obtained, in the state of the art, by means of transport reactions, using subhalogenides of Si having lesser purity. A process known in the literature converts SiI4 with Si, to produce SiI2, at high temperatures; the latter decomposes again at low temperatures, with disproportionation (T. F. Ciszek, T. H. Wang, M. R. Page, R. E. Bauer, M. D. Landry, Conference Record of the IEEE Photovoltaic Specialists Conference (2002), 29th 206-209). Other subhalogenides can also be used for the transport reaction; thus, for example, the use of SiCl2 is particularly described in GB 754554, and the use of SiF2 is particularly described in U.S. Pat. No. 4,070,444 and U.S. Pat. No. 4,138,509.
An economic advantage within a method for the production of high-purity silicon results from the use of inexpensive, easily accessible starting compounds. Preferred products should therefore be the tetrahalogen silanes SiX4, of which SiF4 and SiCl4, in particular, can be produced in cost-advantageous manner. For example, U.S. Pat. No. 4,382,071 describes a method for the characterization of SiF4 from material containing HF and SiO2. SiCl4 can be produced by means of carbochlorination of material containing silicon oxide, according toSiO2+2Cl2+2C→SiCl4+2CO(Examples: EP 0167156 B1, JP 60112610, EP 0302604 B1), and occurs as a by-product in various technical processes, for example in the production of HSiCl3 from silicon and HCl, or within the Siemens process. Another advantage of SiCl4, in particular, is that it is available with great purity, with the purification technique developed to industrial maturity for the Siemens process, just like the HSiCl3 that is processed nowadays.
Furthermore, the greatest possible conversion rates of the starting compounds that contain silicon to elemental silicon, or at least simple re-use, are desirable. If the use of hydrogen as a reduction agent is unavoidable, the hydrogen feed should not lie significantly above what is necessary stoichiometrically, if possible, in order to allow comparatively small gas volumes per silicon produced, on the one hand, and to have to purify as little hydrogen as possible for re-use, on the other hand.
Methods at temperatures above 1000° C. for the characterization of silicon should be avoided, in order to keep the thermal stress on the apparatus low, on the one hand, and to minimize the risk of contamination of the silicon that is produced by contaminants contained in the reactor walls, on the other hand.